The present invention relates to a colored ink ribbon of the electrothermal transfer type for thermal printers. More particularly, the invention relates to a colored ink ribbon of the electrothermal transfer type having excellent gradation recordability to give a continuous change in the optical density of the printed dots, from which a dotted pattern of a hot-melt ink is transferred to recording paper under contact of multi-stylus electrodes from which electric currents are supplied in accordance with the information signals toward a return electrode to evolve Joule's heat and to melt the hot-melt ink to be ready for transfer.
In the recently developed color printers used, for example, in digital color copying machines, video printers, computer graphics and the like, a fully colored image is obtained by the so-called subtractive mixture printing in which superposed printing is performed using transparent colored inks of the three primary colors, i.e. yellow, magenta and cyan, so as to produce seven recording colors of yellow, magenta, cyan, red, blue, green and black according to the method of subtractive mixture by performing changing of the intensity of the picture elements of the three primary colors in 16 steps, 32 steps, 64 steps, and so on. When the gradation of the picture elements of the three primary colors is in 64 steps, for example, the colored images as printed may include 64.sup.3 or about 260,000 different colors.
The color printing by the above mentioned principle can be performed in two different ways including the thermal transfer printing system and the ink-jet printing system. Intensive investigations are now under way to develop practical systems in these two ways. The ink-jet printing sytem has several problems and disadvantages due to the principle of the system in which inks of three different colors are ejected at the surface of the recording paper from three jet nozzles. Namely, the ink-jet printing system cannot give an excellent gradation recordability and the reproducibility of the printed pattern is poor relative to the accuracy of the position. Furthermore, the velocity of printing cannot be high enough taking, for example, 3 minutes or more for printing a full page of an A4 sheet using a line printer.
The thermal transfer printing system can be further classified into two different types including the hot-melt thermal transfer printing system and the sublimate type thermal transfer printing system. The former system utilizes a colored ink ribbon formed of a film such as a polyester film coated with a layer of a hot-melt ink in which a colored pigment is dispersed. The hot-melt ink is melted under heating with a thermal head on the film and directly transferred to the recording paper. In the latter system, a layer of a coloring material containing a sublimable dye and applied to the surface of a film, such as a polyester film, is brought into contact with a sheet of polypropylene-based synthetic paper provided with a developing layer containing a color developer under heating with a thermal head on the film so that the dye sublimed from the layer of the coloring material reaches the developing layer and is developed there to exhibit the color. Although this sublimate type thermal transfer printing system is excellent in the gradation recordability by means of the intensity control of the picture elements and area-controlled gradation can readily be obtained by controlling the diameters of dots so that the system is easily applicable to color printers. But this system is disadvantageous due to the use of special recording paper which is not susceptible to writing with a ball-point pen and the like and the printed color pattern thereon is subject to fading with the lapse of time to greatly decrease the versatility of the system for business use in general. Therefore, the major current of the thermal transfer printing system is toward the hot-melt type.
The hot-melt type thermal transfer printing system has also several problems such as the poor gradation recordability and low printing velocity taking, for example, 60 seconds or more for three-color mixed printing on an A4 sheet using a line printer because of the necessarily slow cycle of the electric power supply for heating up the thermal head in respect of the drawback caused by increasing the number of cycles of the electric power supply that the thermal head is heated up by the accumulation of heat to cause transfer of the ink on extraneous areas resulting in blurred printing. A proposal has been made to solve these problems in the hot-melt type thermal transfer printing system in which melting of the hot-melt ink is effected by local heating with an electric current. Namely, the colored ink ribbon is prepared by coating a film first with a layer of an electrically conductive material having a suitable volume resistivity and then with a layer of a hot-melt ink. The ink ribbon is brought into contact with a return electrode having a broad contacting area, for example by being pressed under multi-stylus electrodes which apply a pulse-wise voltage to the ribbon in accordance with the information signals so that Joule's heat is evolved in the resistance layer in the vicinity of the signaled electrode styluses and the hot-melt ink is melted by this strongly localized heating and transferred to the recording paper. In this system, the heat to melt the hot-melt ink is evolved in the ribbon per se and not in the stylus electrodes so that the frequency of pulse-wise voltage application can be greatly increased to about 3000 pulses/second or even larger from the frequency of about 500 pulses/second in the prior art for heating up the thermal head. This increase in the pulse frequency greatly contributes to the improvement of the printing velocity using a line printer to decrease the time taken for printing on a full page of an A4 sheet, for example, to 10 seconds or less.
The printing ribbon for the electrothermal transfer printing system has a three-layer structure composed of a film of an electro-conductive plastic resin imparted with a volume resistivity of about 1 ohm-cm by compounding with a carbon black filler, a vapor-deposited aluminum layer thereon having a thickness of about 0.1 .mu.m and a top-coat layer of a hot-melt type solid ink for transfer. Namely, the layer of the hot-melt ink is in direct contact with the thin aluminum layer. Therefore, melting of the hot-melt ink cannot take place unless the electric energy converted into heat in the resistance layer exceeds a certain lower limit so that no printing by the transfer of molten ink can be obtained. When the electric energy converted into heat in the resistance layer exceeds the lower limit, the hot-melt ink can of course be melted and transferred to the recording paper but the optical density of the printed dots cannot be increased even by further increasing the electric energy. Therefore, gradation of the picture elements in the printed pattern must be controlled by the binary-area gradation method in which coverage of the printed area is controlled by means of the arrangement of the printed dots. This way of gradation control requires increase in the density of the styluses of the multi-stylus electrode head and an additional electric circuit for the control of the dot matrix in order to obtain a picture image of high resolution and high quality, leading to a disadvantage of increased costs and difficulty in the manufacture of such multi-stylus electrodes in addition to the problem of increased difficulty for designing a compact printing system.